shm_open(2) [osx man page]
SHM_OPEN(2) BSD System Calls Manual SHM_OPEN(2) NAME
shm_open -- open a shared memory object SYNOPSIS
#include <sys/mman.h> #include <fcntl.h> int shm_open(const char *name, int oflag, ...); The parameter "mode_t mode" is optional. DESCRIPTION
The shared memory object referenced by name is opened for reading and/or writing as specified by the argument oflag and the file descriptor returned to the calling process. The returned file descriptor will be the lowest non-open file descriptor for the calling process, and is not shared with any other processes, as it is a new file descriptor. The new file descriptor will have the FD_CLOEXEC flag set. Repeated calls to shm_open with the same string value for name() will return a file descriptor referring to the same shared memory object, provided that the object has not been unlinked by a call to shm_unlink(). The oflag argument may indicate the file is to be created if it does not exist (by specifying the O_CREAT flag), in which case the file is created with mode mode as described in chmod(2) and modified by the process' umask value (see umask(2)). The value of oflag is formed by or'ing the following values: O_RDONLY open for reading only O_RDWR open for reading and writing O_CREAT create object if it does not exist O_EXCL error if create and object exists O_TRUNC truncate size to 0 Exactly one of O_RDONLY or O_RDWR must be specified. If O_TRUNC is specified and the file exists, the file is truncated to zero length. If O_EXCL is set with O_CREAT and the file already exists, shm_open() returns an error. This may be used to implement a simple exclusive access locking mechanism. If successful, shm_open() returns a non-negative integer, termed a file descriptor. It returns -1 and sets errno on failure. The file pointer used to mark the current position within the memory object is set to the beginning of the object. When a new shared memory object is created it is given the owner and group corresponding to the effective user and group of the calling process. There is no visible entry in the file system for the created object in this implementation. When a shared memory object is created, it persists until it it unlinked and all other references are gone. Objects do not persist across a system reboot. The system imposes a limit on the number of file descriptors open simultaneously by one process. getdtablesize(2) returns the current system limit. ERRORS
The named object is opened unless: [EACCES] The required permissions (for reading and/or writing) are denied for the given flags. [EACCES] O_CREAT is specified, the object does not exist, and permission to create the object is denied. [EEXIST] O_CREAT and O_EXCL were specified and the object exists. [EINTR] The shm_open() operation was interrupted by a signal. [EINVAL] The shm_open() operation is not supported. [EMFILE] The process has already reached its limit for open file descriptors. [ENAMETOOLONG] name exceeded the name size limit. This is currently PSHMNAMLEN characters (defined in <sys/posix_shm.h>), but this may change in the future. [ENFILE] The system file table is full. [ENOENT] O_CREAT is not set and the named object does not exist. [ENOSPC] O_CREAT is specified, the file does not exist, and there is insufficient space available to create the object. SEE ALSO
chmod(2), close(2), getdtablesize(2), mmap(2), shm_unlink(2), umask(2) HISTORY
shm_open() is specified in the POSIX Realtime Extension (1003.1b-1993/1003.1i-1995). Darwin August 29, 2008 Darwin
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shm_open(3C) Standard C Library Functions shm_open(3C) NAME
shm_open - open a shared memory object SYNOPSIS
#include <sys/mman.h> int shm_open(const char *name, int oflag, mode_t mode); DESCRIPTION
The shm_open() function establishes a connection between a shared memory object and a file descriptor. It creates an open file description that refers to the shared memory object and a file descriptor that refers to that open file description. The file descriptor is used by other functions to refer to that shared memory object. The name argument points to a string naming a shared memory object. It is unspeci- fied whether the name appears in the file system and is visible to other functions that take pathnames as arguments. The name argument con- forms to the construction rules for a pathname. The first character of name must be a slash (/) character and the remaining characters of name cannot include any slash characters. For maximum portability, name should include no more than 14 characters, but this limit is not enforced. If successful, shm_open() returns a file descriptor for the shared memory object that is the lowest numbered file descriptor not currently open for that process. The open file description is new, and therefore the file descriptor does not share it with any other processes. It is unspecified whether the file offset is set. The FD_CLOEXEC file descriptor flag associated with the new file descriptor is set. The file status flags and file access modes of the open file description are according to the value of oflag. The oflag argument is the bitwise inclusive OR of the following flags defined in the header <fcntl.h>. Applications specify exactly one of the first two values (access modes) below in the value of oflag: O_RDONLY Open for read access only. O_RDWR Open for read or write access. Any combination of the remaining flags may be specified in the value of oflag: O_CREAT If the shared memory object exists, this flag has no effect, except as noted under O_EXCL below. Otherwise the shared memory object is created; the user ID of the shared memory object will be set to the effective user ID of the process; the group ID of the shared memory object will be set to a system default group ID or to the effective group ID of the process. The permission bits of the shared memory object will be set to the value of the mode argument except those set in the file mode creation mask of the process. When bits in mode other than the file permission bits are set, the effect is unspecified. The mode argument does not affect whether the shared memory object is opened for reading, for writing, or for both. The shared memory object has a size of zero. O_EXCL If O_EXCL and O_CREAT are set, shm_open() fails if the shared memory object exists. The check for the existence of the shared memory object and the creation of the object if it does not exist is atomic with respect to other processes executing shm_open() naming the same shared memory object with O_EXCL and O_CREAT set. If O_EXCL is set and O_CREAT is not set, the result is undefined. O_TRUNC If the shared memory object exists, and it is successfully opened O_RDWR, the object will be truncated to zero length and the mode and owner will be unchanged by this function call. The result of using O_TRUNC with O_RDONLY is undefined. When a shared memory object is created, the state of the shared memory object, including all data associated with the shared memory object, persists until the shared memory object is unlinked and all other references are gone. It is unspecified whether the name and shared memory object state remain valid after a system reboot. RETURN VALUES
Upon successful completion, the shm_open() function returns a non-negative integer representing the lowest numbered unused file descriptor. Otherwise, it returns -1 and sets errno to indicate the error condition. ERRORS
The shm_open() function will fail if: EACCES The shared memory object exists and the permissions specified by oflag are denied, or the shared memory object does not exist and permission to create the shared memory object is denied, or O_TRUNC is specified and write permission is denied. EEXIST O_CREAT and O_EXCL are set and the named shared memory object already exists. EINTR The shm_open() operation was interrupted by a signal. EINVAL The shm_open() operation is not supported for the given name. EMFILE Too many file descriptors are currently in use by this process. ENAMETOOLONG The length of the name string exceeds PATH_MAX, or a pathname component is longer than NAME_MAX while _POSIX_NO_TRUNC is in effect. ENFILE Too many shared memory objects are currently open in the system. ENOENT O_CREAT is not set and the named shared memory object does not exist. ENOSPC There is insufficient space for the creation of the new shared memory object. ENOSYS The shm_open() function is not supported by the system. ATTRIBUTES
See attributes(5) for descriptions of the following attributes: +-----------------------------+-----------------------------+ |ATTRIBUTE TYPE |ATTRIBUTE VALUE | +-----------------------------+-----------------------------+ |Interface Stability |Committed | +-----------------------------+-----------------------------+ |MT-Level |MT-Safe | +-----------------------------+-----------------------------+ |Standard |See standards(5). | +-----------------------------+-----------------------------+ SEE ALSO
close(2), dup(2), exec(2), fcntl(2), mmap(2), umask(2), shm_unlink(3C), sysconf(3C), fcntl.h(3HEAD), attributes(5), standards(5) NOTES
Solaris 2.6 was the first release to support the Asynchronous Input and Output option. Prior to this release, this function always returned -1 and set errno to ENOSYS. SunOS 5.11 5 Feb 2008 shm_open(3C)